Measurement of Scattering and Absorption Cross Sections of Microspheres for Wavelengths between 240 nm and 800 nm.
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The fits were excellent from 300 nm to 800 nm.By including the imaginary part obtained from the absorbance measurements below 300 nm, it was possible to obtain a good fit to the observed absorbance data over the region 240 nm to 800 nm.The value of ni at 266 nm was about 0.0060±0.0016 for microspheres with diameters of 1.5 μm, 2.0 μm, and 3.0 μm.
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PubMed Central - PubMed
Affiliation: National Institute of Standards and Technology, Gaithersburg, MD 20899.
ABSTRACT
A commercial spectrometer with a 150 mm integrating sphere (IS) detector was used to estimate the scattering and absorption cross sections of monodisperse polystyrene microspheres suspended in water. Absorbance measurements were performed with the sample placed inside the IS detector. The styrene absorption was non zero for wavelengths less than 300 nm. Correction for fluorescence emission by styrene was carried out and the imaginary part of the index of refraction, ni, was obtained. Absorbance measurements with the sample placed outside the IS detector were sensitive to the loss of photons from the incident beam due to scattering. The absorbance data was fitted with Lorenz-Mie scattering cross section and a correction for the finite acceptance aperture of the spectrometer. The fit parameters were the diameter, the suspension concentration, and the real part of the index of refraction. The real part of the index was parameterized using an expansion in terms of powers of the inverse wavelength. The fits were excellent from 300 nm to 800 nm. By including the imaginary part obtained from the absorbance measurements below 300 nm, it was possible to obtain a good fit to the observed absorbance data over the region 240 nm to 800 nm. The value of ni at 266 nm was about 0.0060±0.0016 for microspheres with diameters of 1.5 μm, 2.0 μm, and 3.0 μm. The scattering cross section, absorption cross section, and the quantum yield at 266 nm of microsphere with a diameter of 2.0 μm was 5.65±0.01 μm(2), 1.54±0.03 μm(2), and 0.027±0.002 respectively. The styrene absorption reduces the scattering cross section by 20 % at 266 nm. No MeSH data available. Related in: MedlinePlus |
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Mentions: The three steps of the analysis culminated in the estimate of the properties of the microspheres and the instrument which yielded a good representation of the measured data. The solid circles in Fig. 8 show the measured absorbance for a suspension of 2.0 μm microspheres divided by the fit parameter c, and the solid trace shows the calculated apparent scattering cross section. The dashed trace in Fig. 8 shows the calculated L-M total scattering cross section for 2.0 μm polystyrene microspheres in water. The calculation of the total scattering cross section (dashed trace) used the same parameters as the calculation of the apparent cross section except for the instrument acceptance angle which was set to zero (this parameter characterizes the instrument and was not needed to calculate the total scattering cross section). Table 1 gives the results of four consecutive independent measurements performed on suspensions of 2.0 μm microspheres. The suspensions were made by pipetting 5 μL of stock suspension (Polysciences Cat. No. 19814, lot no. 625383) into 20 mL distilled water. The values of the cross sections in Table 1 were obtained by using the best fit parameters from Eq. (4) and Eq. (9) and then setting the acceptance angle to zero. The quantum yield (QY) was obtained with additional fluorescence measurements using Eq. (7) and Eq. (8). The real part of the index of refraction was obtained by inserting the fit parameters into Eq. (11) of paper #1. |
View Article: PubMed Central - PubMed
Affiliation: National Institute of Standards and Technology, Gaithersburg, MD 20899.
No MeSH data available.